This invention relates to electronic processing of image information, and more particularly to spectral analysis of acoustic signals used to identify images in tissue and other complex close-spaced structures.
Arteriosclerosis, also known as atherosclerosis, is a common human ailment arising from the deposition of fatty-like substances, referred to as atheromas or plaque, on the walls of blood vessels. Such deposits occur in both the peripheral blood vessels, which feed the limbs of the body, and the coronary vessels, which feed the heart. When deposits accumulate in localized regions of a blood vessel, stenosis, or narrowing of the vascular channel, occurs. As a result, blood flow is restricted and the person's health and life are at great risk.
Numerous approaches for reducing and removing such vascular deposits are known, including balloon angioplasty, in which a balloon-tipped catheter is used to dilate a region of atheroma; atherectomy, in which a blade or cutting bit is used to sever and remove the atheroma; spark gap reduction, in which an electrical spark burns through the plaque; and laser angioplasty, in which laser energy is used to ablate at least a portion of the atheroma. In order to facilitate treatment of stenosis, it is often desirable to obtain a visual image of the interior of the blood vessel within the region of interest. Catheters with imaging elements such as ultrasonic transducers are often used to obtain these images.
In intraluminal ultrasound imaging, the ultrasonic transducers direct ultrasonic pressure waves radially at a target area. These transducers then collect echos from the target area to form an image. The resolution of the images can be increased by utilizing higher frequency ultrasound waves. However, as the frequencies are increased, a significant problem arises.
The ultrasound waves pass through blood to reach the vessel wall tissue. As a result, the transducers also collect undesirable echoes and scattered waveforms from blood cells. In fact, the amount of ultrasound scattering from the blood cells is proportional to the fourth power of the frequency of the ultrasonic wave. As the size of the acoustic wavelength approaches the size of the blood cell, the blood cells start absorbing the energy and radiating the energy back. This problem becomes so significant at higher frequencies that the generated images are practically unusable because the tissue is indistinguishable from the blood.
FIG. 1A illustrates an exemplary ultrasound image according to the prior art. A transducer with a frequency of 20 MHz may be utilized to produce such an image. Regions 102 represent elements that may be of interest to an inspecting user such as a physician. These elements may include atheromas, plaque, and calcium deposits. However, since blood cells have a tendency to scatter some energy, the unprocessed image may also include undesirably bright blood echo regions. Accordingly, the current ultrasound images can include unwanted regions. These undesired regions increase in number and area as higher frequency transducers are utilized.
FIG. 1B illustrates an exemplary unprocessed ultrasound image from a higher frequency transducer. A transducer frequency of 30 MHz and higher may be utilized to produce such an image. Regions 104 represent elements that may be of interest to the user. As shown, the higher frequency transducer creates more regions for the user to inspect. Some of the additional regions are due to the blood echoes. Hence, the user will have a harder task when attempting to identify the true elements of interest such as atheromas, plaque, or calcium deposits.
In the past, the spectrum of interest of an ultrasound image was chosen by selecting from among a group of fixed bandpass filters. Several bandwidths and center frequencies would be chosen in an attempt to match the mechanical parameters of the transducer from which the image information was obtained. The result was a complex and cumbersome process for analyzing the image information. Prior ultrasound image processors include the Ultra instrument system marketed by Scimed Life Systems, Inc. of Maple Grove, Minn.
From the foregoing, it will be readily understood that it is desirable to provide ultrasound imaging over a broad range of frequencies, especially higher ultrasonic frequencies, while eliminating the undesired backscatter from the blood cells.